The effects of moisture on the initial and long-term bonding behaviour of fibre reinforced polymer (FRP) sheets-to-concrete interfaces have been investigated by means of a two-year experimental exposure program. The research is focused on the effects of 1) moisture at the time of FRP installation, in this paper termed "construction moisture", consisting of concrete substratum surface moisture and external air moisture, and 2) moisture, in this paper termed "service moisture", which normally varies throughout the service life of concrete. Concrete beams with FRP bonded to their soffits were prepared. Before bonding, concrete substrates were pre-conditioned with different moisture contents and treated with different primers. The FRP bonded concrete beams were then cured under different humidity conditions, before being subjected to combined wet/dry and thermal cycling regimes to accelerate the exposure effects. Adhesives with different elastic modulus were used to investigate the long-term durability of each adhesive, when subjected to accelerated wet/dry cycling. Pull-off tests and bending tests were conducted at the beginning of the cycling and then again, after 8 months, 14 months and 2 years of exposure so as to evaluate the tensile and shear performance of the FRP-to-concrete interfaces. It was found that the effect of the concrete substrate moisture content on short-term interfacial bond performance could be eliminated if an appropriate primer was used. All FRP-to-concrete bonded joints failed at the interface between the primer and concrete after exposure while those not exposed usually failed within the concrete substrate. After exposure to an environment of accelerated wet/dry cycles, it was also found that the interfacial tensile bond strength degraded asymptotically with the exposure time while the flexural capacity of the FRP sheet bonded plain concrete beams even increased. The mechanism behind the above, which is an apparently contradictory phenomenon, is discussed.

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